1. Field of the Invention
The present invention relates to a fibre-type light conversion device.
2. Description of Background Information
An optical pickup is known by which high density writing and reading of information on and from a disc are enabled by changing the wavelength of a laser beam emitted from a laser source to a half the original wavelength by means of a light conversion device (reference is directed to Japanese Patent Application Laid-Open No. 61-50122).
As the light conversion device for use in this type of pickup, there is an optical fibre-type SHG (Second Harmonics Generator) using a second-order non-linear optical effect. A phase matching, of (Cerenkov radiation system, is adopted in the optical fibre type SHG. With this system, it is possible to generate a second harmonic wave (referred to as SH wave hereinafter) whose phase matching condition is attained almost automatically. The general concept of this device is shown in FIGS. 1A and 1B.
In FIG. 1A, when the fundamental wave mode is propagated through the core with the effective reflective index of N (.omega.), the non-linear polarizing wave generating the SH wave is also propagated at the same phase velocity C/N (.omega.) (C is the speed of light). It is assumed that this non-linear polarizing wave produces the SH wave in a direction making an angle .theta. with respect to the direction of the wave guide at a point A, and generates the SH wave at the angle .theta., as before, at a point B, after the elapse of a unit time. If the SH wave generated at the point A propagates through the cladding and reaches point C after the elapse of the unit time, and the angle .theta. is such that lines AC and BC are perpendicular to each other, then the plane of the SH wave which is generated from the non-linear polarized wave between A and B becomes equal to BC, and as a result, a coherent SH wave is generated.
The condition of the phase matching is, according to the figure, as follows: EQU N(.omega.)=N.sub.clad (2.omega.) cos.theta. (1)
where N.sub.clad (2.omega.) is the refractive index of the clad for the SH wave.
This in turn gives, EQU N(.omega.)&lt;N.sub.clad (2.omega.) (2)
The above equation means that the SH is generated automatically in the direction where the phase matching is performed when at least the condition mentioned by the equation (2) is satisfied. Generally, with the refractive indices of the clad and core for the fundamental wave being n.sub.clad (.omega.) and n(.omega.) and with air as the over-layer, the condition for the fundamental wave to propagate through the core as a mode is expressed as follows: EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n(.omega.) (3)
Wavelength dispersion of the clads, refractive index will now be considered. Since n.sub.clad (.omega.)&lt;n.sub.clad (2.omega.), the equation (2) is satisfied for all of the fundamental wave modes irrespectively of the diameter of the core so far as the following expression (4) is satisfied. EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n.sub.clad (2.omega.) (4)
Moreover, there are fundamental modes satisfying the equation (2) in a certain range of the diameter of the core even under the following condition. EQU N.sub.clad (.omega.)&lt;n.sub.clad (2.omega.)&lt;n(.omega.)
The second harmonic wave generated in this way is propagated in a clad mode as illustrated in FIG. 1B in which total reflection occurs repeatedly at the boundary between the clad and air. Then, the second harmonic wave is emitted in conical shape from the end of fibre in directions making an angle .theta. relative to the fibre's direction. The equiphase front of the second harmonic wave emitted in this way is in a conical surface with an axis on the central axis of the fibre.
As a light source for this fibre-type SHG, a laser beam is utilized for example, and the laser beam is derived as an incidental light beam through an optical system consisting of a plurality of lenses. Since the non-linear sensitivity of the non-linear optical crystal in the core relates to the efficiency of the generation of the SH wave in the case of the fibre-type SHG, it is necessary to set the non-linear optical crystal so that its non-linear polarization generated by the non-linear sensitivity of the non-linear light is optimized with respect to the light beam from light source having a definite polarization. Therefore, conventionally, the direction of the non-linear optical crystal is determined to optimize the non-linear polarization generated by the non-linear sensitioity of the non-linear crystal by rotating the fibre-type SHG around its optical axis.
However, as a result of the rotation of the fibre-type SHG, errors in alignments such as the center of rotation (the position of the core) are likely to be generated each the fibre-type SHG is rotated. Therefore it has been difficult to set the direction of the fibre-type SHG at a direction in which a maximum efficiency is obtained.